Osteomyelitis is a common and debilitating infection of bone that affects healthy children and adults, as well as those with comorbidities such as diabetes and musculoskeletal trauma. Bacterial pathogens, notably Staphylococcus aureus, are the most common causes of osteomyelitis. Treatment options for bone infections are limited by both the increasing prevalence of multi-drug resistant bacterial pathogens, as well as pathogen- induced changes in bone remodeling that limit antibiotic penetration into the infection site. Even with prolonged administration of appropriate antimicrobial therapy, patients suffering from osteomyelitis often experience significant morbidity, including bone fractures, deep venous thrombosis, and septicemia. Osteomyelitis therefore necessitates aggressive interventions such as surgical debridement, after which some patients still progress to chronic infection. The mechanisms by which bacterial pathogens induce and sustain osteomyelitis, trigger detrimental changes in bone remodeling, and evade host immune responses in the bone are poorly understood. Likewise, the host immune responses that protect bone from osteomyelitis, or that contribute to pathogen-induced changes in bone remodeling have not been fully delineated. Finally, how bone homeostasis is modulated by infectious and inflammatory signals is not well defined. The goals of this proposal are to understand how bacterial virulence factors perturb bone homeostasis (Aim 1), to delineate host immune responses that either promote clearance of bacterial pathogens from bone, or contribute to pathogen-induced changes in bone remodeling (Aim 2), and to define bacterial factors that are critical for survival within the bone (Aim 3). Successful completion of the proposed Aims will significantly enhance an understanding of the pathogenesis of osteomyelitis, define mechanisms governing bone homeostasis during infection and inflammation, and meet the need for new therapies that treat osteomyelitis and counteract pathogen-induced changes in bone remodeling. Dr. James Cassat is currently a Clinical Fellow in Pediatric Infectious Diseases at Vanderbilt University Medical Center. He completed the M.D. and Ph.D. degrees at the University of Arkansas for Medical Sciences prior to his clinical training at Vanderbilt. During clinical fellowship, Dr. Cassat has focused his research efforts on understanding the pathogenesis of osteomyelitis, one of the most common invasive bacterial infections in children. Dr. Cassat has created innovative tools to model the host-pathogen interface during osteomyelitis, including a new animal model that utilizes high resolution tomographic imaging to quantify changes in bone remodeling during osteomyelitis. These tools, recently published in Cell Host and Microbe, enabled generation of substantial preliminary data for the studies outlined in this application. In completion of the proposed Aims, Dr. Cassat will draw upon greater than 10 years of experience studying the molecular pathogenesis of S. aureus, but will also gain new proficiencies in translational imaging modalities, immunology, advanced flow cytometry, and bone biology. The compilation of these skills will facilitate Dr. Cassat's development into an independently-funded pediatric physician-scientist, and will ultimately enable a translational research career that addresses an important clinical problem while seeking to define the pathways that govern musculoskeletal homeostasis in the setting of infection and inflammation. His professional development will be guided by an inter-disciplinary scholarship oversight committee, chaired by his mentor Dr. Eric Skaar. Dr. Skaar is an internationally recognized expert in host-pathogen interactions, with a specific focus on the important human pathogens S. aureus, Acinetobacter baumannii, and Bacillus anthracis. Additional members of Dr. Cassat's scholarship oversight committee will facilitate the acquisition of new techniques and knowledge throughout the award period, while fostering important collaborative efforts. These members include experts in the innate and adaptive immune responses to human pathogens (Dr. John Williams and Dr. Buddy Creech), translational imaging modalities (Dr. Charles Manning), and fundamental bone biology (Dr. Florent Elefteriou). Completion of the proposed studies will require a multi-disciplinary approach that capitalizes on Vanderbilt's strengths in the study of host-pathogen interactions, translational imaging sciences, immunology, and bone biology. The outstanding resources of the Division of Pediatric Infectious Diseases, The Vanderbilt Center for Bone Biology, and the Vanderbilt University Institute of Imaging Sciences will provide Dr. Cassat a unique and stimulating environment for professional development. In total, Vanderbilt is the ideal environment for completion of the proposed studies.